Interpolation for use in channel estimation

ABSTRACT

A method for the estimation of the transfer function of a transmission channel in a receiving system of UMTS type envisages the computation of a plurality of channel coefficients, included among known channel coefficients corresponding to pilot symbols, through the reiteration of an interpolation algorithm, capable of calculating an intermediate point (Z, f(Z)) between a first extreme and a second extreme of a determined interval, the first extreme being formed by at least two known points and the second extreme being formed by at least one known point, the intermediate point to be calculated having as abscissa (Z) the abscissa value of the mean point between the points defining the interval rounded off to the integer closest to the first extreme, and having as ordinate (F(Z)) the arithmetic average between the ordinate of the known point of the second extreme and the ordinate of a point, chosen between the two known points of the first extreme, having a distance from the intermediate point equal to the distance between the intermediate point and the known point of the second extreme.

The above-identified application is the US phase of PCT ApplicationPCT/EP03/02773 filed 17 Mar. 2003, claiming the priority of the EuropeanApplication 02425177.9 itself filed on 21 Mar. 2002.

TECHNICAL FIELD

The present invention is related to telecommunications systems and inparticular it concerns a method and a device for the estimation of thetransfer function of a transmission channel.

BACKGROUND ART

As is well-known, one of the most used techniques in the mobile radiocommunication systems is the W-CDMA (Wideband-Code Division MultipleAccess) technique, by virtue of its high spectral efficiency as comparedto other multiple access techniques. Within said technique and inparticular in the FDD (Frequency Division Duplexing) mode differentsolutions have been adopted to increase system capacity. Among saidsolutions it is worth mentioning the coherent tracking of the signalreceived by the radio mobile station or mobile terminal.

This type of signal tracking requires a very accurate estimation of thetransfer function of the transmission channel, referenced to in thesequel as “channel estimation”, in the presence of fading and multiplereflections due to user's motion in the covering area, without givingany details about a user's displacement velocity.

Like other system functions, also the channel estimation function may beassigned to processing devices (for example, DSPs, microcontrollers,etc) and in such a case this is achieved by means of software or throughdedicated devices (for example FPGAs, ASICs, etc.) and therefore theimplementation modality is mainly hardware based. During the definitionof the telecommunications system architecture, the design engineer hasto find out the best distribution of the different tasks amongprogrammed devices and wired devices.

Thus, the different architectures achievable must then be analysed interms of computational burden for a DSP device, traffic volume over thebus dedicated to data transfer, and the optimum distribution amonghardware and software resources.

Considering by way of an example a complex system such as a modem for aUMTS base station or mobile terminal, it is evident that thecommunication bus between the DSP device and the hardware resources actsas a bottleneck of the system. For instance, as to a radio base station,the load of the bus becomes greater and greater as the number of usersincreases, and when the functions involved require the transfer betweenDSP and hardware resources of considerable bulks of data.

Channel estimation is one of said functionalities, since it has to berepeated for any fingers (sub-channels) of each individual user.

Let us consider the worst case in which the maximum number of usersNu=128 are communicating within a cell and each user has a finger numberNf=8 for each Rake receiver (a receiver which is typical for example ofUMTS base stations). The quantity of data to be transferred for eachfinger in a time slot (time interval) of duration Ts=666 μs may beestimated to be equal to a number of bits NB=400. As a consequence,there will be a considerable data stream over the bus, namely:Nb·Nf·Nu/Ts=400·8·128/666·10−6=615 Mbit/s

As of today, various algorithms capable of performing the channelestimation are known; among them there are for instance those describedin the document EP 0 912 019, wherein two different interpolationmethods, the one of linear type, the other through Kalman filters, areused, the method described in the document EP 1 032 168, wherein theinterpolation is performed with the method of Lagrange's polynomials,and that described in the document U.S. Pat. No. 5,886,911, in which useis made of the bi-section method.

The solutions described in the above documents have however somedrawbacks and are not directly applicable to UMTS systems, such as thebi-section method, or are extremely complex for an implementation ofhardware type.

Taking into account as a matter of fact, the considerable data streamalready present on the bus of a base station, it should be preferred notto further overload such a bus, since this would require complexinterpolation operations within the estimation of the channel, such asthe operations necessary to perform an interpolation by the method ofLagrange's polynomials.

The device can be fully implemented through hardware and therefore canbe easily integrated into a Rake receiver for base stations, keeping thesame performance as of the software solutions based on DSP processors.Since no data transfer with the DSP processor is required, thecommunication bus is left free for other tasks.

A particular subject matter of the present invention are a method and adevice for the estimation of the transfer function of a transmissionchannel, as described in the appended claims.

SUMMARY OF THE INVENTION

The method and the device subject matter of this invention make use ofan algorithm of a low complexity, for the estimation of the transferfunction of a transmission channel, suitable for both the transmissionpath toward a base station, called “up-link”, and for the transmissionpath toward a mobile terminal, called “down-link”.

The method according to the invention envisages the computation of aplurality of channel coefficients, included among known channelcoefficients corresponding to pilot symbols, through the reiteration ofan interpolation algorithm, capable of calculating an intermediate pointbetween a first extreme and a second extreme of a determined interval.The first extreme is formed by at least two known points and the secondextreme is formed by at least one known point.

The intermediate point to be calculated has an abscissa equal to theabscissa value of a mean point between the points defining the intervalrounded off to the integer closest to the first extreme, and has anordinate equal to the arithmetic average between the ordinate of theknown point of the second extreme and the ordinate of a point, chosenbetween the two known points of the first extreme, having a distancefrom the intermediate point equal to the distance between theintermediate point and the known point of the second extreme.

BRIEF DESCRIPTION OF DRAWINGS

This and other characteristics of this invention will become evidentfrom the following description of a preferred embodiment of the same,given by way of a non-limiting example, and from the attached drawings,wherein:

FIG. 1 is a schematic representation of the symbols of a data channelDPDCH and of a control channel DPCCH, in the case of a transmission pathtoward a radio mobile station;

FIG. 2 is a graph illustrating a plurality of channel coefficientsobtained through linear interpolation between two known channelcoefficients.

FIG. 3 is a flow chart illustrating a first interpolation algorithmimplemented according to this invention;

FIG. 4 is a schematic representation illustrating a sequence ofoperations for computing the channel coefficients through the algorithmof FIG. 3, when N_(PILOT)=3;

FIG. 5 is a schematic representation illustrating a sequence ofoperations for computing the channel coefficients through the algorithmof FIG. 3, when N_(PILOT)=4;

FIG. 6 is a schematic representation illustrating a sequence ofoperations for computing the channel coefficients through the algorithmof FIG. 3, when N_(PILOT)=5;

FIG. 7 is a schematic representation illustrating a sequence ofoperations for computing the channel coefficients through the algorithmof FIG. 3, when N_(PILOT)=6;

FIG. 8 is a schematic representation illustrating a sequence ofoperations for computing the channel coefficients through the algorithmof FIG. 3, when N_(PILOT)=7;

FIG. 9 is a schematic representation illustrating a sequence ofoperations for computing the channel coefficients through the algorithmof FIG. 3, when N_(PILOT)=8;

FIG. 10 is a state diagram summarising the operations required forcomputing the channel coefficients in the cases depicted in FIGS. 4 to9;

FIG. 11 is a schematic representation illustrating a sequence ofoperations for computing the channel coefficients through a secondalgorithm implemented according to the present invention, whenN_(PILOT)=6; and

FIG. 12 is a block diagram of a circuit performing an interpolationalgorithm, implemented according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method and the device according to the present invention will now bedescribed in detail with reference to the UMTS (Universal MobileTelecommunications System) systems, relating to the up-link path in theFDD mode.

The coherent tracking for the radio interface of UMTS systems may beobtained by using time-multiplexed pilot symbols in the control channelDPCCH (Dedicated Physical Control Channel).

In FIG. 1 there is schematically depicted the time relation between thesymbols of the data channel DPDCH (Dedicated Physical Data ControlChannel) and the symbols of control channel DPCCH in a time intervalcalled “time slot”. In the case illustrated in said figure, there are atthe beginning of the slot three pilot symbols, X_(Q)(0), X_(Q)(1) andX_(Q) (2), followed by other symbols used for the control, in particularfour symbols TFCI (Transport-Format Combination Indicator), two FBI(Feedback Information) symbols and a TPC (Transmit Power Control)symbol.

Pilot symbols X_(Q)(k), with k=0, 1, 2, . . . , N_(PILOT−1) (whereN_(PILOT) is the number of pilot symbols in the slot) are also known tothe receiver which utilises such information jointly with the receivedsymbols Z(k), in order to evaluate the channel effect on each pilotsymbol X_(Q)(k).

By carrying out the complex product of the received symbol Z(k) by thecorresponding pilot symbol XQ(k) one obtains the channel coefficients,in particular the in phase and quadrature components of the productcorrespond to the components C_(Q)±C_(I) of the channel coefficientsC(k)=C_(I)(k)+jC_(Q)(k).

It becomes therefore evident that it is possible to estimate the channelcoefficients only in relation of the pilot symbols transmitted overchannel DPCCH. However, since the pilot symbols are not transmittedcontinuously but only in coincidence with the initial part of a timeslot, performing the channel estimation requires the computation,through some form of interpolation, of the channel coefficientscorresponding to the missing pilot symbols.

The number of pilot symbols within an individual slot may vary from aminimum of three to a maximum of eight; therefore the number of channelcoefficients that will have to be estimated through interpolation, shallvary between seven and two, being ten the total number of symbolspresent in a slot of a control channel DPCCH.

FIG. 2 depicts for instance the case in which seven channel coefficients(in the figure are represented the in the phase components only),globally indicated with reference 2, have been computed throughinterpolation between the last coefficient C_(I)(N_(PILOT)−1) of thecurrent slot, corresponding to a value of abscissa value A and a firstcoefficient of the following slot, indicated in the figure as C_(I)(10)and corresponding to a value of abscissa B. The pilot symbols, thus alsothe channel coefficients, always correspond by common assumption tointeger abscissa values (0, 1, 2, 3, . . . ) on a horizontal time axis(Time).

The interpolation method and device implemented according to the presentinvention allow computation of the components C_(I)(k)±C_(Q)(k) of thechannel coefficients without using complex operations such asmultiplication and division, but only utilising additions and divisionsby two (the latter ones of easy implementation in hardware through ashift operation on the right side of a register) allowing a considerablereduction in the hardware architecture complexity of the interpolationunit.

The interpolation method according to the present invention will now bedescribed in detail with reference to the flow chart of FIG. 3, Thealgorithm shown in FIG. 3 allows the computation, through interpolationof a plurality of points, included between a last channel coefficient ofabscissa A, a current slot L, and a first channel coefficient ofabscissa B, of a slot L+1 subsequent to said current slot. Since theminimum number of channel coefficients in each slot is equal to three,the coefficient, of abscissa A−1, immediately preceding the last channelcoefficient of each slot is always known and therefore can be used forthe interpolation computation.

Within the algorithm of FIG. 3 some variables, X, Y, Z, W are used withthe following meaning:

X: abscissa of the known left-hand point of the interpolation interval,a value initially corresponding to A;

Y: abscissa of the known right-hand point, used for the interpolation, avalue initially corresponding to B;

Z: abscissa of an intermediate point between X and Y;

W: abscissa of the extreme left point, actually used for theinterpolation, corresponding, depending on the case, to point X or topoint X−1;

f(k): ordinate corresponding to a generic abscissa k.

Variables X e Y are initialised as being equal to the extremes A and Bin the initial block indicated by reference 32 in FIG. 3.

Subsequently in block 36 of such figure, abscissa Z is obviouslycalculated as an integer of the intermediate point between X an Y. Thevalue of abscissa Z is rounded off to the lower integer through functionFLOOR (symbolically indicated in the figure). In the same block thecomputation is made of the abscissa of the extreme left-hand point W,which will be equal either to X or to X−1, depending on whether thevalue of X+Y is odd or even, respectively (by applying the formulaW=2·Z−Y). Then the ordinate f(Z) of the intermediate point Z iscalculated by arithmetically averaging between the ordinate of theextreme left-hand point W and the ordinate of the right-hand extreme Y,which are known.

Then, still within block 36, a check is made on whether the justcalculated point on abscissa Z corresponds to the abscissa point X+1. Ifsuch a condition is not met, value Z is assigned to abscissa Y of theknown right-hand point, and again operations contained in block 36 areapplied. In practice Block 36 algorithm is recursively applied to thehalf-interval at the left side of the point previously computed, untilthe abscissa point X+1 is attained.

Subsequently, one should consider the operations indicated as a wholewith reference 38, operating by increasing abscissas, starting from thelast computed point X+1, and searching a first point still to becalculated. Once such a point has been found, the operations containedin block 40 are carried out or one terminates the algorithm if theright-hand extreme B of the interpolation interval has already beenreached, for all the points have already been computed.

If however a point has been found which is still to be computed andwhose abscissa is equal to the value taken by variable Z, the next stepis to assign to variable X the value Z−1 that represents the first knownleft-hand point, and to search, by subsequent attempts, for the firstknown right-hand point of Z, that is. the value which is assigned tovariable Y. To perform the latter operation, variable Y is increased bya unit each time, until a corresponding known coefficient has beenreached. These operations are carried out within block 40

With these new values of X and Y, the recursive algorithm of block 36 isagain applied, and in cascade use is made of the procedure containedwithin block 38 for the search of a new point to be computed. If all thepoints have been computed, the algorithm terminates; otherwise a new X−Yinterval is determined to which again recursive algorithm 36 is toapply.

The previously-mentioned algorithm, shown in FIG. 3, allows computation,exclusively by sums, divisions by two and compare operations betweenregisters, of any number of channel coefficients contained in aninterval defined on the left side by two known coefficients (of abscissaA and A−1) and on the right side by a known coefficient (of abscissa B).

For a better understanding of the general interpolation algorithmdescribed above, we will now consider its application to six particularcases, typical of the UMTS system, from the case in which the pilotsymbols (N_(PILOT)) are three, and the coefficients to be computed areseven, up to the case where the pilot symbols are eight and the channelcoefficients to be computed are two only.

The fist case (N_(PILOT)=3) is schematically illustrated in FIG. 4. Onthe abscissa the channel coefficients are plotted corresponding to thethree pilot symbols of the current slot, namely to abscissa positions 0,(A−1) and 2 (A), and to the first pilot symbol of the subsequent slot,to the abscissa position 10 (B) respectively. The channel coefficientsto be computed are therefore those corresponding to the abscissapositions 3 to 9.

The interpolation operation is carried out by subsequent steps; thenumber of steps directly depends upon the number of channel coefficientsto be computed. In case of FIG. 4 the computation is performed in sevensteps (Step 1 to Step 7).

At Step 1, the algorithm of block 36 of flow chart of FIG. 3 is appliedby assigning to variables X and Y the values A (which is worth 2) and B(which is worth 10), respectively:X=A=2Y=B=10

Thus the following is obtained:

Z=FLOOR [(2+10)/2]=6; abscissa of the intermediate point to be computed(denoted by letter C in the Figure).

W=2·6−10=2; abscissa of the left-hand extreme point, corresponding inthis case to X.

Once the abscissa of the extreme left-hand point W, of the extremeright-hand point Y and of the intermediate point Z, of which theordinate is to be computed, are known, the next operation is tocalculate the value of the corresponding coefficientC(6)=C_(I)(6)+jC_(Q)(6), computing the components C_(I) e C_(Q) by thearithmetic averaging rule (point C on the graph of FIG. 4):C _(I)(6)=[C _(I)(2)+C _(I)(10)]/2andC _(Q)(6)=[C _(Q)(2)+C _(Q)(10)]/2

After checking that abscissa Z of the computed point, in this case beingequal to 6, still does not correspond to the point X+1=3, one assignsthe value Z=6 to variable Y and determine a new coefficient (point D,step 2):X=2; Y=6Z=FLOOR [(2+6)/2]=4W=2·4−6=2C _(I)(4)=[C _(i)(2)+C _(i)(6)]/2andC _(Q)(4)=[C _(Q)(2)+C _(Q)(6)]/2

Also in this case the output condition from block 36 is not met, sinceZ=4 is different from X+1=3; thus the next step is to compute a newcoefficient (point E, step 3):X=2; Y=Z=4Z=FLOOR [(2+4)/2]=3W=2·3−4=2C _(I)(3)=[C _(I)(2)+C _(I)(4)]/2andC _(Q)(3)=[C _(Q)(2)+C _(Q)(4)]/2

At this point a verification is made of condition Z=X+1 which allowspassing from block 36 to block 38.

By applying the rules described within block 38, one finds the firstpoint still to be computed on the right hand of the point of abscissaZ=3 is the abscissa point 5, then one realises that said point doescorrespond not to the right hand extreme B=10 and go on to block 40,having assigned value 5 to the variable Z.

Within block 40 one determines the first known left-hand point X=Z−1=4and the first known right-hand Y=6, and with these values of X and Y onethen goes back to block r 36 to calculate a new coefficient (point F,step 4):X=4; Y=6Z=FLOOR [(4+6)/2]=5W=2·5−6=4C _(I)(5)=[C _(I)(4)+C _(I)(6)]/2andC _(Q)(5)=[C _(Q)(4)+C _(Q)(6)]/2

In this case the condition Z=X+1 is immediately verified, and one goeson again to block 38 where the first right-hand point still to becomputed is the abscissa point Z=7. Within block 40, the first knownleft-hand point X=Z−1=6 and the first known right-hand point Y=10 aredetermined, and with these values of X e Y one goes back to block 36, tocompute a new coefficient (point G, step 5):X=6; Y=10Z=FLOOR [(6+10)/2]=8W=2·8−10=6C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2andC _(Q)(8)[C _(Q)(6)+C _(Q)(10)]/2

Condition Z=X+1 is not verified; thus one remains within block 36 tocalculate a new coefficient (point H, step 6):X=6; Y=Z=8Z=FLOOR [(6+8)/2]=7W=2·7−8=6C _(I)(7)=[C _(I)(6)+C _(I)(8)]/2andC _(Q)(7)=[C _(Q)(6)+C _(Q)(8)]/2

Having reached point Z=X+1=7, one goes on to block 38 where it must beverified whether the right-end extreme has not yet been reached, and adetermination of abscissa Z=9 of the next point to be calculated is tobe effected. Within block 40 the first known left-hand point X=Z−1=8 andthe first known right-hand point Y=10 will then be determined, and withthese values of X and Y we go back to block 36 to compute a newcoefficient (point I, step 7).X=8; Y=10Z=FLOOR [(8+10)/2]=9W=2·9−10=8C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2andC _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2

Condition Z=X+1 has now been verified; therefore we go on to block 38where variable X is increased to reach the value Z=10 which, ascorresponding to the right-hand extreme B, leads to termination of theinterpolation algorithm (step “STOP” in FIG. 3).

As may be derived from the above equations, all the channel coefficientshave been calculated only by sums and divisions by two, i.e. operationswhich are easily carried out directly on hardware.

In the case (N_(PILOT)=3). described above, the known coefficient ofabscissa A−1 has not been used, since the distance between points A andB is a power of 2, in particular 2³=8. This condition is brought aboutalso in the case N_(PILOT)=7, where such distance is 2²=4.

The algorithm depicted in FIG. 3 allows however the computation of allthe intermediate coefficients also in cases when such a distance is nota power of 2, as in the remaining cases, illustrated in the sequel.

In FIG. 5 there is depicted the case in which the pilot symbols are four(N_(PILOT)=4) and the coefficients to be calculated are six, from C(4)to C(9). The interpolation plan illustrated in FIG. 5, as will be appearfrom the following detailed explanation, uses the abscissa point A−1 asthe left-hand extreme for the interpolation. Even if it introduces aslight degradation of the overall characteristics of the system, thisapproximation greatly simplifies the computation of the interpolatedcoefficients.

Let us now see how the algorithm of FIG. 3 is applied to the presentcase (N_(PILOT)=4).

As the first step, use is made the first time of algorithm of block 36of FIG. 3 flow chart, by assigning the values of A (which is worth 3)and of B (which worth 10), to variables X and Y, respectively:X=A (A=3)Y=B (B=10)

Thus:

Z=FLOOR [(3+10)/2]=6; abscissa of the intermediate point to becalculated (denoted in the figure by letter C);

W=2·6−10=2; abscissa of the extreme left-hand point, in this case itdoes not correspond to X.

Once the abscissas are known of the extreme left-hand point W, theextreme right-hand point Y and the intermediate point Z of which theordinate is to be calculated, one starts computing the value of thecorresponding coefficient C(6)=C_(I)(6)+jC_(Q)(6), calculatingcomponents C_(I) e C_(Q) by the arithmetic averaging rule (point C inthe graph of FIG. 5):C _(I)(6)=[C _(I)(2)+C _(I)(10)]/2andC _(Q)(6)=[C _(Q)(2)+C _(Q)(10)]/2

After verifying that abscissa Z of the calculated point, in this caseequal to 6, does not still correspond to point X+1=4, one assigns thevalue Z=6 to variable Y, and a new coefficient (point D, step 2) iscomputed:X=3; Y=6Z=FLOOR [(3+6)/2]=4W=2·4−6=2C _(I)(4)=[C _(I)(2)+C _(I)(6)]/2andC _(Q)(4)=[C _(Q)(2)+C _(Q)(6)]/2

In this case the output condition of block 36 is verified, since Z=4 isequal to X+1=4; consequently one goes on to block 38.

By applying the rules described within block 38, one finds that thefirst point still to be calculated on the right hand of abscissa Z=4, isthe abscissa point 5, verifies that such point does not correspond tothe right-hand extreme B=10, and goes on within block 40 having assignedthe value 5 to variable Z.

Within block 40, one will determine the first known left-hand X=Z−1=4and the first known right hand point Y=6, and with these values of X andY around block 36 where a new coefficient (point E, step 3) is computed.X=4; Y=6Z=FLOOR [(4+6)/2]=5W=2·5−6=4C _(I)(5)=[C _(I)(4)+C _(I)(6)]/2andC _(Q)(5)=[C _(Q)(4)+C _(Q)(6)]/2

At this point the condition Z=X+1 must be verified that allows passingfrom block 36 to block 38.

By applying the rules described within block 38, one finds that thefirst point still to be calculated on the right-hand of abscissa pointZ=5 is the abscissa point 7, verifies that such point does notcorrespond to the right-hand extreme B=10, and goes on within block 40,having assigned the value 7 to variable Z.

Within block 40, the first known left-hand point X=Z−1=6 and the firstknown right-hand point Y=10 are determined, and with these values of Xand Y one returns to block 36 to calculate a new coefficient (point F,step 4):X=6; Y=10Z=FLOOR [(6+10)/2]=8W=2·8−10=6C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2andC _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2

Condition Z=X+1 is not verified, therefore one remains within block 36to calculate a new coefficient (point G, step 5):X=6; Y=Z=8Z=FLOOR [(6+8)/]=7W=2·7−8=6C _(I)(7)=[C _(I)(6)+C _(I)(8)]/2andC _(Q)(7)=[C _(Q)(6)+C _(Q)(8)]/2

In this case condition Z=X+1 is immediately verified, and one returns toblock 38 and finds out that the first right-hand point still to becalculated is the abscissa point Z=9. Within block 40 the first knownleft-hand point X=Z−1=8 and the first known right-hand point Y=10 aredetermined, and with these values of X and Y one returns to block 36 tocompute a new coefficient (point H, step 6):X=8; Y=10Z=FLOOR [(8+10)/]=9W=2·9−10=8C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2andC _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2

Condition Z=X+1 is now verified; therefore one goes on to block 38 wherevariable Z is increased passing to value Z=10, which, corresponding tothe right-hand extreme Y, leads to terminate the interpolation algorithm(step STOP in FIG. 3).

FIG. 6 illustrates the case where each slot contains five pilot symbols(N_(PILOT)=5), and the number of channel coefficients to be calculatedis five. This operation is performed using both coefficient C(4)corresponding to both the last pilot symbol and coefficient C(3)corresponding to the last but one pilot symbol. Also in this case theapproximation introduces a slight degradation in the overallcharacteristics of the system; however, the computation of theinterpolated coefficients is on the other hand greatly simplified.

The five steps required for the calculation of the five coefficientswill be now described in a shortened way, giving the values taken by thevariables, since the application methodology of the algorithm of FIG. 3is equivalent to the one described with reference to the previous cases.

As a first step, coefficient C(7) (point C, step 1) is calculated:X=4; Y=10Z=FLOOR [(4+10)/2]=7W=2·7−10=4C _(I)(7)=[C _(I)(4)+C _(I)(10)]/2andC _(Q)(7)=[C _(Q)(4)+C _(Q)(10)]/2

In the step 2, coefficient C(5) (point D, step 2) is calculated:X=4; Y=7Z=FLOOR [(4+7)/2]=5W=2·5−7=3C _(I)(5)=[C _(I)(3)+C _(I)(7)]/2andC _(Q)(5)=[C _(Q)(3)+C _(Q)(7)]/2

In step 3, coefficient C(6) (point E, step 3) is calculated:X=5; Y=7Z=FLOOR [(5+7)/2]=6W=2·6−7=5C _(I)(6)=[C _(I)(5)+C _(I)(7)]/2andC _(Q)(6)=[C _(Q)(5)+C _(Q)(7)]/2

In step 4, coefficient C(8) (point F, step 4) is calculated:X=7; Y=10Z=FLOOR [(7+10)/2]=8W=2·8−10=6C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2andC _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2

In step five, coefficient C(9) (point G, step 5) is calculated:X=8; Y=10Z=FLOOR [(8+10)/2]=9W=2·9−10=8C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2andC _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2

Also in this case the five coefficients C(5) . . . C(9) have beenobtained exclusively performing additions and divisions by two.

The case in which each slot contains six pilot symbols (N_(PILOT)=6) andthe number of channel coefficients to be calculated is equal to four, isinstead illustrated in FIG. 7. Also in this case use is made of both thecoefficient C(5), corresponding to the last pilot symbol, and thecoefficient C(4), corresponding to the last-but-one pilot symbol.

The interpolation plan of FIG. 7 is based on four consecutive steps,summarised hereinafter:

As a first step, coefficient C(7) (point C, step 1) is calculated:X=5; Y=10Z=FLOOR [(5+10)/2]=7W=2·7−10=4C _(I)(7)=[C _(I)(4)+C _(I)(10)]/2andC _(Q)(7)=[C _(Q)(4)+C _(Q)(10)]/2

In step 2, 1 coefficient C(6) (point D, step 2) is calculated:X=5; Y=7Z=FLOOR [(5+7)/2]=6W=2·6−7=5C _(I)(6)=[C _(I)(5)+C _(I)(7)]/2andC _(Q)(6)=[C _(Q)(5)+C _(Q)(7)]/2

In step 3, coefficient C(8) (point E, step 3) is calculated:X=7; Y=10Z=FLOOR [(7+10)/2]=8W=2·8−10=6C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2andC _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2

In step 4, coefficient C(9) (point F, step 4) is calculated:X=8; Y=10Z=FLOOR [(8+10)/2]=9W=2·9−10=8C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2andC _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2

Also in this case the operations are carried out exclusively throughadditions and divisions by two.

In FIG. 8 there is illustrated the case in which the slot contains sevenpilot symbols (N_(PILOT)=7), and the number of channel coefficients tobe calculated is equal to three. The operation is performed in threesteps using the C(6) coefficient corresponding to the last pilot symbol.

As a first step, coefficient C(8) (point C, step 1) is calculated:X=6; Y=10Z=FLOOR [(6+10)/2]=8W=2·8−10=6C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2andC _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2

In step 2, coefficient C(7) (point D, step 2) is calculated:X=6; Y=8Z=FLOOR [(6+8)/2]=7W=2·7−8=6C _(I)(7)=[C _(I)(6)+C _(I)(8)]/2andC _(Q)(7)=[C _(Q)(6)+C _(Q)(8)]/2

In step 3, coefficient C(9) (point E, step 3) is calculated:X=8; Y=10Z=FLOOR [(8+10)/2]=9W=2·9−10=8C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2andC _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2

In FIG. 9 there is instead illustrated the last case in which each slotcontains eight pilot symbols (N_(PILOT)=8) and the number of channelcoefficients to be calculated is equal to two. This operation is carriedout in two steps using coefficient C(6) which corresponds, in this case,to the last-but-one pilot symbol.

As a first step, coefficient C(8) (point C, step 1) is calculated:X=6; Y=10Z=FLOOR [(6+10)/2]=8W=2·8−10=6C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2andC _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2

In step 2, coefficient C(9) (point D, step 2) is calculated:X=8; Y=10Z=FLOOR [(8+10)/2]=9W=2·9−10=8C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2andC _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2

Also in this case the fact of using the channel coefficientcorresponding to the last but one pilot symbol introduces a slightdegradation in the overall system characteristics, however thecomputation of the interpolated coefficients is greatly simplified.

The six cases previously described and illustrated with reference toFIGS. 4 to 9, can be schematised in the state diagram shown in FIG. 10.The diagram of FIG. 10, whose sequence of operations essentially dependson the value of the N_(PILOT) parameter, shows how it is possible toimplement, by means of a simple state machine, a hardware or softwaredevice, which is the embodiment of the method of this invention. In thediagram there is shown only the phase component C_(I)(k) of the channelcomponents, owing to the fact that the formulas for the computation ofthe corresponding quadrature component C_(Q)(k) are equivalent.

In FIG. 11 there is shown instead an example of application, referred tothe case N_(PILOT)=6, of a second method of interpolation implementedaccording to the present invention. In the example of FIG. 11, for thecomputation of a plurality of channel coefficients, corresponding to theabscissa positions 6 to 9, use is made of the known channelcoefficients, corresponding to the last pilot symbol, of abscissa A, ofthe current slot L and to the first two pilot symbols, of abscissas Band B+1, of the slot L+1 subsequent to the current one.

To implement this second interpolation method, it is necessary to storean additional pilot symbol with respect to the cases previouslydescribed, namely the one of abscissa 11 (B+1), corresponding to thesecond pilot symbol of the subsequent slot.

To explain the sequence of steps illustrated in the application exampleof FIG. 11, and how the same method can be applied also to the remainingcases, from N_(PILOT)=3 to N_(PILOT)=8, we will now analyse in detailthe second interpolation method, implemented according to the presentinvention.

This second method allows the computation, by interpolation, of aplurality of points comprised between a last channel coefficient, ofabscissa A, of a current slot L, and the first two channel coefficients,of abscissas B and B+1, of a subsequent slot L+1.

By way of example, use will be made within this algorithm of somevariables that have the same meaning of the variables previously usedwith reference to the algorithm of FIG. 3:

X: abscissa of the known left-hand point, used for the interpolation, avalue which initially corresponds to A;

Y: abscissa of the known right-hand point of the interpolation interval,a value which initially corresponds to B;

Z: abscissa of an intermediate point between X and Y;

W: abscissa of the right-hand extreme point, actually used for theinterpolation corresponding from time to time either to point Y or topoint Y+1;

f(k): ordinate corresponding to a generic abscissa k.

The variables X and Y are initially set to be equal to the extremes Aand B.

Subsequently, it goes on calculating the abscissa Z, obviously meant asan integer number, of the intermediate point between X and Y. The valueof abscissa Z is rounded off to the higher integer through the functionCEIL [(X+Y)/2]. Then calculation is made of the abscissa of theright-hand extreme point D, which is found to be equal to Y or Y+1,depending on whether the value of X+Y is even or odd, respectively, bymeans of the formula W=2•Z−X. The calculation is then carried out of theordinate f(Z) of intermediate point Z by arithmetic averaging betweenthe ordinate of the right-hand extreme point W and the ordinate of theleft-hand extreme X, which are known.

Then the check is made on whether the point just calculated, of abscissaZ, corresponds to the point of abscissa Y−1. If this condition is met,the value of Z is assigned to variable X of the known left-hand point,and the operations of the preceding paragraph are applied again. Inpractice the above operations are recursively applied to eachhalf-interval on the right-hand of the point previously calculated untilthe abscissa point Y−1 is reached.

Subsequently the procedure applied is by decreasing abscissas startingfrom the last calculated point Y−1 and searching for a first point stillto be computed. If the left-hand extreme A of the interval is reached,the algorithm is terminated, since all the points have already beencalculated.

If however a point still to be calculated has been found, the abscissaof which is equal to the value taken by variable Z, then the variable Yis assigned the value Z+1, which represents the first known right-handpoint, and the search is made by subsequent attempts for the first knownleft-hand point of Z, a value which is assigned to variable X. Toperform the latter operation, variable X is decreased each time by aunit, until a corresponding known coefficient is reached.

With the new values of X and Y, use is made again of the recursivealgorithm for computing an intermediate point of abscissa Z and incascade for the search procedure of a new point, still to be calculated.When all the points have been computed, the algorithm is terminated;otherwise a new interval X−Y is determined, to which the above recursivealgorithm is to be applied.

Let us now analyse how the algorithm previously described is applied tothe interpolation plan of FIG. 11.

As a first step, coefficient C(8) (point C, step 1) is calculated:X=A=5; Y=B=10Z=CEIL [(5+10)/2]=8W=2·8−5=11C _(I)(8)=[C _(I)(5)+C _(I)(11)]/2andC _(Q)(8)=[C _(Q)(5)+C _(Q)(11)]/2

In step 2, coefficient C(9) (point D, step 2) is calculated:X=8; Y=10Z=CEIL [(8+10)/2]=9W=2·9−8=10C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2andC _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2

In step 3, coefficient C(7) (point E, step 3) is calculated:X=5; Y=8Z=CEIL [(5+8)/2]=7W=2·7−5=9C _(I)(7)=[C _(I)(5)+C _(I)(9)]/2andC _(Q)(7)=[C _(Q)(5)+C _(Q)(9)]/2

In step 4, coefficient C(6) (Point F, step 4) is calculated:X=5; Y=7Z=FLOOR [(5+7)/2]=6W=2·6−5=7C _(I)(6)=[C _(I)(5)+C _(I)(7)]/2andC _(Q)(6)=[C _(Q)(5)+C _(Q)(7)]/2

The algorithm described in the second interpolation method may beregarded as a “mirror” version of the algorithm previously describedwith reference to FIG. 3. As a matter of fact, this method, too, allowscomputations exclusively through additions, divisions by two and compareoperations between registers, of any number of channel coefficientscontained in an interval defined at the left side by a known coefficient(of abscissa A) and at the right side by two known coefficients (ofabscissas B and B+1).

The two interpolation methods described above can be used in combinationto implement a third method that might be advantageous in terms of speedof execution.

In the event in which there are available as known extremes of theinterpolation interval two channel coefficients on the left-hand, ofabscissa A−1 and A, and two known channel coefficients on theright-hand, of abscissa B and B+1, it is possible to apply “in parallel”both interpolation methods described above, in a left-hand sub-intervaland in a right-hand sub-interval of the original interval, respectively.In fact it sufficient to calculate, as a first step, an average point Cbetween the two extremes, A and B, of the interval, rounding up, ifnecessary, the abscissa of such point C to the next lower integer or tothe next higher integer, indifferently, in order to define a left-handsub-interval comprised among the abscissa points A−1, A and C and aright-hand sub-interval comprised among the abscissa points C and B,B+1. At this point it is possible to apply in parallel, by betterexploiting therefore the hardware resources involved, the firstinterpolation method in the left-hand sub-interval and the secondinterpolation method in the right hand sub-interval, thus performing inparallel the computation of all the coefficients comprised in theinterval.

Likewise it is possible to foresee an additional variation of themethods implemented according to the present invention; for instance itpossible to apply for the interpolation, in place of the known channelcoefficient immediately preceding the last known coefficient of thecurrent slot or immediately following the first known coefficient of thesubsequent slot, other known coefficients farther from the interpolationinterval or a linear combination of the same.

As an example of use of a linear combination of known coefficients thecase of three pilots (N_(PILOT)−3) is analysed. First a linearcombination is performed of the three known pilots of the left handgroup.CL _(I) =C _(I)(2)/2+[C _(I)(0)+C _(I)(1)]/4CL _(Q) =C _(Q)(2)/2+[C _(Q)(0)+C _(Q)(1)]/4,and the result is assigned to coefficient C(2):C_(I)(2)=CL_(I)C_(Q)(2)=CL_(Q)

Also Coefficient C(1) is calculated as a linear combination of the samepilots:CL _(I) =C _(I)(1)/2+[C _(I)(0)+C _(I)(2)]/4CL _(Q) =C _(Q)(1)/2+[C _(Q)(0)+C _(Q)(2)]/4

The result is assigned to coefficient C(1).C_(I)(1)=CL_(I)C_(Q)(1)=CL_(Q)

A similar linear combination may be calculated on the basis of thepilots of the subsequent slot.CL _(I) =C _(I)(10)/2+[C _(I)(11)+C _(I)(12)]/4CL _(Q) =C _(Q)(10)/2+[C _(Q)(11)+C _(Q)(12)]/4,and then assigned to coefficient C(10)C_(I)(10)=CL_(I)C_(Q)(10)=CL_(Q)

Linear combinations are so chosen as to be implemented in the form ofsums and shifts of bits, in a similar way as has been illustrated forthe interpolation.

The interpolation algorithm for the calculation of the missingcoefficients can therefore be applied following the plan alreadyillustrated for three pilots and reported in FIG. 4. This variation ofthe method is applicable in the extremely frequent case of the pilotsbeing affected by noise: a linear combination, which represents aweighed average of the known channel coefficients, albeit affected bynoise, provides a more reliable estimate of the known points for theinterpolation.

The block diagram of FIG. 12 shows a possible hardware implementation ofa device for the channel estimation according to the present invention,in case of a transmission path towards a radio base station and aspreading factor of the channel DPDCH equal to 256.

The device of FIG. 12, called CEU (Channel Estimation Unit), allowsextension of the channel estimation to channel coefficients subsequentto the pilot symbols, by implementing one of the interpolation methodspreviously described.

The CEU unit receives at its input the channel symbols received afterperforming the operations of “descrambling”, “despreading” with the codeof channelling DPCCH and integration. The received pilot symbols whichare supplied to the CEU unit are denoted by Z_(i)(k), where subscript“i” indicates the signal component (i=I for the phase component or i=Qfor the quadrature component).

The first operation performed by the CEU unit is the multiplication ofthe received pilot symbols by the reference pilot symbols X_(Q) (k). Asa matter of fact, the reference pilot symbols are known to the receiver.Subsequently it is necessary to invert the sign of the phase component.The sign inversion of the phase component is effected throughmultiplication by factor M₁, setting M₁=−1 for the phase component andM₁=+1 for the quadrature component, as shown in the diagram of FIG. 12.

Once the above operations have been carried out, the effect of thesequence of the pilot symbols is removed and the complex values obtainedrepresent only the phase displacement introduced by the channel. Thesevalues are stored into memory 100 through the input denoted in FIG. 10as Input_port_2.

When all the pilot symbols of the current slot and one or two initialpilot symbols, of the subsequent slot, depending on the method to beapplied, have been received and stored into memory 100, the process ofinterpolation begins under the control of the CLU unit 102 (CLU=ControlLogic Unit).

The logic control unit 102 is a finite state machine (FSM) that performsa predefined sequence of operations, as a function of input parameterN_(PILOT) (corresponding to the number of pilot symbols present in thecurrent slot). For instance, in case of the interpolation methodpreviously described with reference to FIG. 3, the logic control unit102 carries out the sequence of operations shown in the state diagram ofFIG. 10.

The basic operation effected by the logic control unit 102 is dividedinto the following three steps:

-   -   reading the first operand from memory 100 and loading it into        the first register 104;    -   reading the second operand from memory 100 and loading it into        the second register 106;    -   writing the arithmetic average of the two operands into memory        100 through its Input_port_1.

The computation of the arithmetic average of the two operands requiresan addition and a division by two; the sum is effected in adder 108 andthe division by two in block 110 which carries out a right-hand shift,discarding in practice the less significant bit (LSB) of the dataresulting from adder 108.

All the calculations to be carried out only require sums and right-handshifts, that is operations which are easily implementable on hardware.

When all channel coefficients C_(i)(k), with k ranging from 0 to 9, havebeen computed, they are sequentially read from the memory and suppliedto the CCU unit (Channel Compensation Unit), not shown in FIG. 10, sinceits function is well known to the man skilled in the art.

The multiplying factor shown in FIG. 12 as M₂ is required to invert thesign of the quadrature component and to provide the complex conjugate ofthe channel estimations to the channel compensation unit (CCU). M2 isequal to +1 for the phase component and to a −1 for the quadraturecomponent.

1. An iterative method of estimating channel coefficients byinterpolation between known channel coefficients, the coefficients beingidentified by integer abscissa values on a time axis, the knowncoefficients comprising at least two coefficients with adjacent abscissavalues A−1 and A at the left side of an interval and at least onecoefficient with abscissa value B at the right of the interval, whereinone iteration of the method comprises calculating an abscissa value asz=FLOOR[(A+B)/2], and calculating the coefficient with abscissa z as thearithmetic mean of the coefficients with abscissae values A and B, ifA+B is even, and as the arithmetic mean of the coefficients withabscissae values A−1 and B, if A+B is odd, the coefficient with abscissaz constituting a known coefficient for any further iterations.
 2. Themethod according to claim 1 wherein said channel coefficients to becalculated are comprised between a first known channel coefficient, ofabscissa A, corresponding to a last pilot symbol of a current slot (L)and a second known channel coefficient, of abscissa B, corresponding toa first pilot symbol of a slot (L+1) subsequent to said current slot, athird channel coefficient of abscissa A-i being on the left-hand of saidfirst channel coefficient of abscissa A, and the computation of saidchannel coefficients being carried out by the following steps: a)carrying out a first iteration in an interval defined by the knownchannel coefficients of abscissa A and B in which a first intermediatecoefficient is calculated and performing subsequent iterations insub-intervals defined each time on the left-hand by said known channelcoefficient of abscissa A and on the right-hand by the intermediatecoefficient and calculated in the preceding iteration, until theabscissa point A+1 is reached and computed; b) searching, by increasingabscissas, for a first point, still to be calculated, on the right-handof the last intermediate coefficient calculated; defining as extremes ofa new interval having on the left side the first known left-hand pointand on the right side the first known right-hand point with respect tosaid point still to be calculated; and further recursively performingfurther iterations of the method in said new interval by carrying outsubsequent iteration in sub-intervals defined from time to time by theintermediate coefficient calculated in the preceding iteration, untilthe point immediately adjacent to the left-hand extreme of said newinterval is reached and calculated; and c) repeating step b) until thechannel coefficient associated to the value of abscissa B−1 iscalculated.
 3. Method according to claim 2, wherein each slot containsthree pilot symbols (0, 1, 2), said first known channel coefficient ofabscissa A is the coefficient C(2)=C_(I)(2)+C_(Q)(2) corresponding tothe last pilot symbol (2) of the current slot (L), said second knownchannel coefficient of abscissa B is the coefficientC(10)=C_(I)(10)+jC_(Q)(10) corresponding to the first pilot symbol (10)of a subsequent slot (L+1), and said third known channel coefficient ofabscissa A−1 is the coefficient C(1)=C_(I)(1)+C_(Q)(1) corresponding tothe last but one pilot symbol (1) of the current slot (L) and thecomputation of channel coefficients C(k)=C_(r)(k)+C _(Q)(k), with k=3 .. . 9, is performed according to the following sequence, C_(I)(k) beingthe in phase component of the channel coefficient C(k) and C_(Q)(k)being the quadrature component of the channel coefficient C(k):C _(I)(6)=[C _(I)(2)+C _(I)(10)]/2; C _(Q)(6)=[C _(Q)(2)+C _(Q)(10)]/2;C _(I)(4)=[C _(I)(2)+C _(I)(6)]/2; C _(Q)(4)=[C _(Q)(2)+C _(Q)(6)]/2;C _(I)(3)=[C _(I)(2)+C _(I)(4)]/2; C _(Q)(3)=[C _(Q)(2)+C _(Q)(4)]/2;C _(I)(5)=[C _(I)(4)+C _(I)(6)]/2; C _(Q)(5)=[C _(Q)(4)+C _(Q)(6)]/2;C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2; C _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2;C _(I)(7)=[C _(I)(6)+C _(I)(8)]/2; C _(Q)(7)=[C _(Q)(6)+C _(Q)(8)]/2;C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2; C _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2.4. Method according to claim 2, wherein each slot contains four pilotsymbols (0, 1, 2, 3), said first known channel coefficient of abscissa Ais the coefficient C(3)=C_(I)(3)+jC_(Q)(3) corresponding to the lastpilot symbol (3) of the current slot (L), said second known channelcoefficient of abscissa B is the coefficient C(10)=C_(I)(10)+C_(Q)(10)corresponding to the first pilot symbol (10) of a subsequent slot (L+1),and said third known channel coefficient of abscissa A−1 is thecoefficient C(2)=C_(I)(2)+C_(Q)(2) corresponding to the last but onepilot symbol (2) of the current slot (L), and the computation of thechannel coefficients C(k)=C_(I)(k)+jC_(Q)(k), with k=4 . . . 9, isperformed according to the following sequence, C_(I)(k) being the inphase component of the channel coefficient C(k) and C_(Q)(k) being thequadrature component of the channel coefficient C(k):C _(I)(6)=[C _(I)(2)+C _(I)(10)]/2; C _(Q)(6)=[C _(Q)(2)+C _(Q)(10)]/2;C _(I)(4)=[C _(I)(2)+C _(I)(6)]/2; C _(Q)(4)=[C _(Q)(2)+C _(Q)(6)]/2;C _(I)(5)=[C _(I)(4)+C _(I)(6)]/2; C _(Q)(5)=[C _(Q)(4)+C _(Q)(6)]/2;C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2; C _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2;C _(I)(7)=[C _(I)(6)+C _(I)(8)]/2; C _(Q)(7)=[C _(Q)(6)+C _(Q)(8)]/2;C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2; C _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2.5. Method according to claim 2, wherein each slot contains five pilotsymbols (0, 1, 2, 3, 4), said first known channel coefficient ofabscissa A is the coefficient C(4)=C_(I)(4)+jC_(Q)(4) corresponding tothe last pilot symbol (4) of current slot (L), said second known channelcoefficient of abscissa B is the coefficient C(10)=C_(I)(10)+jC_(Q)(10)corresponding to the first pilot symbol (10) of a subsequent slot (L+1),and said third known channel coefficient of abscissa A−1 is thecoefficient C(3)=C_(I)(3)+jC_(Q)(3) corresponding to the last but onepilot symbol (3) of the current slot (L), and the computation of thechannel coefficients C(k)=C_(I)(k)+jC_(Q)(k), with k=5 . . . 9, isperformed according to following sequence, C_(I)(k) being the in phasecomponent of the channel coefficient C(k) and C_(Q)(k) being thequadrature component of the channel coefficient C(k):C _(I)(7)=[C _(I)(4)+C _(I)(10)]/2; C _(Q)(7)=[C _(Q)(4)+C _(Q)(10)]/2;C _(I)(5)=[C _(I)(3)+C _(I)(7)]/2; C _(Q)(5)=[C _(Q)(3)+C _(Q)(7)]/2;C _(I)(6)=[C _(I)(5)+C _(I)(7)]/2; C _(Q)(6)=[C _(Q)(5)+C _(Q)(7)]/2;C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2; C _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2;C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2; C _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2.6. Method according to claim 2, wherein each slot contains six pilotsymbols (0, 1, 2, 3, 4, 5), said first known channel coefficient ofabscissa A is the coefficient C(5)=C_(I)(5)+jC_(Q)(5) corresponding tothe last pilot symbol (5) of the current slot (L), said second knownchannel coefficient of abscissa B is the coefficientC(10)=C_(I)(10)+C_(Q)(10) corresponding to the first pilot symbol (10)of a subsequent slot (L+1), and said third known channel coefficient ofabscissa A−1 is the coefficient C(4)=C_(I)(4)+jC_(Q)(4) corresponding tothe last but one pilot symbol (4) of the current slot (L), and thecomputation of the channel coefficients C(k)=C_(I)(k)+jC_(Q)(k), withk=6 . . . 9, is performed according to following sequence, C_(I)(k)being the in phase component of the channel coefficient C(k) andC_(Q)(k) being the quadrature component of the channel coefficient C(k):C _(I)(7)=[C _(I)(4)+C _(I)(10)]/2; C _(Q)(7)=[C _(Q)(4)+C _(Q)(10)]/2;C _(I)(6)=[C _(I)(5)+C _(I)(7)]/2; C _(Q)(6)=[C _(Q)(5)+C _(Q)(7)]/2;C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2; C _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2;C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2; C _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2.7. Method according to claim 2, wherein each slot contains seven pilotsymbols (0, 1, 2, 3, 4, 5, 6), said first known channel coefficient ofabscissa A is the coefficient C(6)=C_(I)(6)+jC_(Q)(6) corresponding tothe last pilot symbol (6) of the current slot (L), said second knownchannel coefficient is the coefficient C(10)=C_(I)(10)+jC_(Q)(10)corresponding to the first pilot symbol (10) of a subsequent slot (L+1),and said third known channel coefficient of abscissa A−1 is thecoefficient C(5)=C_(I)(5)+jC_(Q)(5) corresponding to the last but onepilot symbol (5) of the current slot (L), and the computation of thechannel coefficients C(k)=C_(I)(k)+jC_(Q)(k), with k=7 . . . 9, isperformed following the sequence, C_(I)(k) being the in phase componentof the channel coefficient C(k) and C_(Q)(k) being the quadraturecomponent of the channel coefficient C(k):C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2; C _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2;C _(I)(7)=[C _(I)(6)+C _(I)(8)]/2; C _(Q)(7)=[C _(Q)(6)+C _(Q)(8)]/2;C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2; C _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2.8. Method according to claim 2, wherein each slot contains eight pilotsymbols (0, 1, 2, 3, 4, 5, 6, 7), said first known channel coefficientof abscissa A is the coefficient C(7)=C_(I)(7)+jC_(Q)(7) correspondingto the last pilot symbol (7) of the current slot (L), said second knownchannel coefficient of abscissa B is the coefficientC(10)=C_(I)(10)+jC_(Q)(10) corresponding to the first pilot symbol (10)of a subsequent slot (L+1), and said third known channel coefficient ofabscissa A−1 is the coefficient C(6)=C_(I)(6)+jC_(Q)(6) corresponding tothe last but one pilot symbol (6) of the current slot (L), and thecomputation of the channel coefficients C(k)=C_(I)(k)+jC_(Q)(k), withk=8, 9, is performed according to the sequence, C_(I)(k) being the inphase component of the channel coefficient C(k) and C_(Q)(k) being thequadrature component of the channel coefficient C(k):C _(I)(8)=[C _(I)(6)+C _(I)(10)]/2; C _(Q)(8)=[C _(Q)(6)+C _(Q)(10)]/2;C _(I)(9)=[C _(I)(8)+C _(I)(10)]/2; C _(Q)(9)=[C _(Q)(8)+C _(Q)(10)]/2.9. The method according to claim 1, wherein said channel coefficients tobe calculated are comprised between two known left-hand channelcoefficients (A+1, A) corresponding to the last two pilot symbols of acurrent slot (L), and two known right-hand channel coefficients (B, B+1)corresponding to the first two pilot symbols of a slot (L+1) subsequentto said current slot, and the computation of said channel coefficientsis performed by applying the first time iterative method of claim 16 forcalculating an intermediate coefficient, thus dividing into twosub-intervals the interval comprised between said known left-handchannel coefficients and said known right hand channel coefficients, andby subsequently applying in parallel to said the iterative method ofclaim 16 for computing the remaining channel coefficients comprised ineach of said sub-intervals.
 10. The method according to claim 1 whereinat least one known point of said first or second extreme is a pointwhich has been obtained through a linear combination of known channelcoefficients.
 11. The method according to claim 1 wherein saidcommunications network is a radio mobile telecommunications network ofUMTS type.
 12. A device for the estimation of the transfer function of atransmission channel in a receiving system for a telecommunicationsnetwork, the device comprising: a memory means for storing channelcoefficients corresponding to a current slot (L) and at least onechannel coefficient corresponding to a slot (L+1) subsequent to saidcurrent slot (L); interpolation means for reading from said memory meansfirst and second operands corresponding to known channel coefficientsand for writing into said memory means a value corresponding to thearithmetic average between said first and second operand, said valuecorresponding to a new channel coefficient; logic control means foraddressing in reading and writing (R/W) said memory means and forcontrolling said interpolation means so as to perform through individualinterpolation operations the computation and the storage into suchmemory means of individual channel coefficients, said logic controlcarrying out a series of interpolation operations according to themethod described in claim
 1. 13. A radio base station of the typecomprising a Rake receiver for receiving signals coming from mobileterminals and equipped with a device for the estimation of the transferfunction of a transmission channel through the computation of aplurality of channel coefficients, the estimation of the transferfunction being performed according to the method described in claim 1.14. A mobile terminal of the type comprising a receiver for thereception of signals coming from a radio base station and equipped witha device for the estimation of the transfer function of a transmissionchannel through the computation of a plurality of channel coefficients,the estimation of the transfer function being performed according to themethod described in claim
 1. 15. An iterative method of estimatingchannel coefficients by interpolation between known channelcoefficients, the coefficients being identified by integer abscissavalues on a time axis, the known coefficients comprising at least onecoefficients with adjacent abscissa values A at the left side of aninterval and at least two coefficients with abscissa values B and B+1 atthe right of the interval, wherein one iteration of the method comprisescalculating an abscissa value as z=CEIL[(A+B)/2], and calculating thecoefficient with abscissa z as the arithmetic mean of the coefficientswith abscissae values A and B, if A+B is even, and as the arithmeticmean of the coefficients with abscissae values A and B+1, if A+B is odd,the coefficient with abscissa z constituting a known coefficient for anyfurther iterations.
 16. The method according to claim 15 wherein saidchannel coefficients to be calculated are comprised between a firstknown channel coefficient of abscissa A, corresponding to a last pilotsymbol of a current slot (L), and a second known channel coefficient ofabscissa B, corresponding to a first pilot symbol of a slot (L+1)subsequent to said current slot, a third channel coefficient of abscissaB+1 being on the right hand of said first channel coefficient ofabscissa B, and the computation of said channel coefficients isperformed by the steps of: a) carrying out a first iteration in theinterval defined by the known channel coefficients of abscissa A and Bin which a first intermediate coefficient is calculated and performingsubsequent iterations in sub-intervals defined from time to time on theright-hand by said known channel coefficient of abscissa B and on theleft-hand by the intermediate coefficient calculated in the precedingiteration, until the abscissa point B−1 is reached and calculated; b)searching, by decreasing abscissas, for a first point still to becalculated on the left-hand of the last intermediate coefficientcalculated; defining a new interval having on the left side the firstknown left hand point and on the right side the first known right-handpoint with respect to said point still to be calculated; and recursivelycalculating further iterations of the method in said new interval bycarrying out subsequent iterations in sub-intervals defined from time totime by the right hand extreme of said new interval and by a left handextreme formed by the intermediate coefficient derived in the previousiteration, until the point immediately adjacent to the right handextreme of said new interval is reached and calculated; and c) repeatingstep b) until the channel coefficient associated to the value ofabscissa A+1 is calculated.
 17. A device for the estimation of thetransfer function of a transmission channel in a receiving system for atelecommunications network, the device comprising: a memory means forstoring channel coefficients corresponding to a current slot (L) and atleast one channel coefficient corresponding to a slot (L+1) subsequentto the current slot (L); interpolation means for reading from the memorymeans first and second operands corresponding to known channelcoefficients and for writing into the memory means a value correspondingto the arithmetic average between the first and second operand, thevalue corresponding to a new channel coefficient; logic control meansfor addressing in reading and writing (R/W) the memory means and forcontrolling the interpolation means so as to perform through individualinterpolation operations the computation and the storage into suchmemory means of individual channel coefficients, the logic controlcarrying out a series of interpolation operations according to themethod described in claim
 15. 18. A radio base station of the typecomprising a Rake receiver for receiving signals coming from mobileterminals and equipped with a device for the estimation of the transferfunction of a transmission channel through the computation of aplurality of channel coefficients, the estimation of the transferfunction being performed according to the method described in claim 15.19. A mobile terminal of the type comprising a receiver for thereception of signals coming from a radio base station and equipped witha device for the estimation of the transfer function of a transmissionchannel through the computation of a plurality of channel coefficients,the estimation of the transfer function being performed according to themethod described in claim 15.